Capacitors are elements that are used extensively in semiconductor devices for storing an electrical charge. Capacitors essentially comprise two conductive electrodes separated by an insulator. The capacitance, or amount of charge held by the capacitor per applied voltage, depends on a number of parameters such as the area of the electrodes, the distance between the electrodes, and the dielectric constant value for the insulator between the electrodes, as examples.
It is common for capacitors, as well as resistors, transistors, diodes, and other circuit elements, to be formed in semiconductor integrated circuits (IC's) of various types. Capacitors formed within analog integrated circuit fabrications typically assure proper operation of those analog integrated circuits, for example. Capacitors formed within digital integrated circuits typically provide storage locations for individual bits of digital data. Capacitors are used in filters, analog-to-digital converters, memory devices, control applications, and many other types of semiconductor devices.
One type of capacitor is a PIP capacitor, which is frequently used in mixed-mode devices and logic devices, as examples. PIP capacitors are used to store a charge in a variety of semiconductor devices. PIP capacitors are often used as a storage node in a memory device, for example. A PIP capacitor is typically formed horizontally on a semiconductor wafer, with two polysilicon electrodes sandwiching a dielectric layer parallel to the wafer surface.
A PIP capacitor formed within an integrated circuit usually comprises a double layer polysilicon capacitor. Double layer polysilicon capacitors are formed from two substantially planar conductive polysilicon electrodes separated by a dielectric layer. Double layer polysilicon capacitors provide several advantages when used within integrated circuits. For example, double layer polysilicon capacitors may easily be formed within several locations within an IC.
Performance enhancements may also make blending logic and memory on the same IC attractive for particular applications. In electronic systems in which logic and memory are packaged separately, data signals between the two may have to pass through several layers of packaging (i.e. through the original IC chip to external pins, then through the card and/or board wiring, and finally into the receiving IC chip, including its internal wiring), all of which cause undesirable propagation delays. As device sensitivities increase and device size decreases, transistor switching speeds no longer limit the logic delay or access time of the IC. Rather, the time required for the device to charge capacitive loads becomes the limiting factor for IC performance. The capacitive load is partially dependent on the length of lines interconnecting devices, thus minimizing these connection lengths, such as through combining logic and memory on the same IC, will enhance performance.
It is also desirable to integrate passive components such as capacitors with the logic and memory (active components) on the same IC, referred to in the art as mixed-mode devices. As described above, capacitors are a basic building block in many electronic circuits, and may be used for analog applications such as switched capacitor filters, for example. Traditionally, formation of a capacitor requires two separate lithography masks: a first mask for the bottom electrode and a second mask for the top electrode. Lithography masks are expensive and are thus a significant cost factor in IC fabrication.
Therefore, there are trends in the semiconductor manufacturing industry to develop methods of minimizing the number of lithography masks used, and also to develop methods of integrating mixed-mode capacitors with memory cells, such as split gate flash transistor processes.
U.S. Pat. No. 6,277,686 issued to Yeh et al. on Aug. 21, 2001, which is hereby incorporated herein by reference, describes a method of forming a mixed mode process PIP capacitor for split gate flash devices. Yeh et al. recognize that it is undesirable for a PIP capacitor to have a thick poly-oxide layer formed over the first or bottom electrode because the thick poly-oxide layer causes the unit capacitance of the PIP capacitor to be low. In this patent, Yeh et al. teach a processing method wherein a first mask is used to pattern a first polysilicon layer to form a floating gate of a split gate flash memory device, and a second mask is used to pattern the first polysilicon layer to form a PIP capacitor bottom electrode. This manufacturing method is costly because two lithography masks are required to pattern the first polysilicon layer.